Subterranean deposits of coal, also referred to as coal seams, contain substantial quantities of entrained methane gas. Production and use of methane gas from coal deposits has occurred for many years. Substantial obstacles, however, have frustrated more extensive development and use of methane gas deposits in coal seams.
For example, one problem of production of gas from coal seams may be the difficulty presented at times by over-balanced drilling conditions caused by low reservoir pressure and aggravated by the porosity of the coal seam. During both vertical and horizontal surface drilling operations, drilling fluid is used to remove cuttings from the well bore to the surface. The drilling fluid exerts a hydrostatic pressure on the formation which, when exceeding the pressure of the formation, can result in a loss of drilling fluid into the formation. This results in entrainment of drill cuttings in the formation, which tends to plug the pores, cracks, and fractures that are needed to produce the gas.
Certain methods are available to drill in an under-balanced state. Using a gas such as nitrogen in the drilling fluid reduces the hydrostatic pressure, but other problems can occur, including increased difficulty in maintaining a desired pressure condition in the well system during drill string tripping and connecting operations.